1. Technical Field
The present invention relates to an elemental layer on a packaging substrate and the method and apparatus for applying the elemental layer. More specifically, the invention disclosed herein pertains to an ultra-thin inorganic metal oxide layer that serves as an oxygen and water vapor barrier layer and/or to serve as an interface for future functionalization when applied to a packaging substrate. This layer can be formed during the manufacture of the packaging substrate or in later processing stages by use of known chemical vapor deposition apparatus and methods in a commercial packaging substrate manufacturing context.
2. Description of Related Art
Multi-layered packaging substrates made from petroleum-based products, polymers, copolymers, bio-polymers and/or paper structures are often used where there is a need for advantageous barrier, sealant, and graphics-capability properties. Barrier properties in one or more layers comprising the packaging substrate are important in order to protect the product inside the package from light, oxygen and/or moisture. Such a need exists, for example, for the protection of foodstuffs that may run the risk of flavor loss, staling, or spoilage if sufficient barrier properties are not present to prevent transmission of light, oxygen, or moisture into or out of the package. A graphics capability may also be required so as to enable a consumer to quickly identify the product that he or she is seeking to purchase, which also allows food product manufacturers a way to label information such as the nutritional content of the packaged food, and present pricing information, such as bar codes, to be placed on the product.
In the packaged food industry, protecting food from the effects of moisture and oxygen is important for many reasons, including health, safety, and consumer acceptability (i.e., preserving product freshness and taste). Conventional methods to protect food contents incorporate specialized coatings or layers within or on a surface of the packaging substrate, which function as an impervious barrier to prevent the migration of light, water, water vapor, fluids and foreign matter. These coatings may consist of coextruded polymers (e.g., ethyl vinyl alcohol, polyvinyl alcohol, polyimides, polyamides (i.e. nylons and polyvinyl acetate) and/or a thin layer of metal or metal oxide, depending on the level of barrier performance required to preserve the quality of the product stored within the package volume.
Coatings produced by chemical vapor deposition are known to provide certain barrier characteristics to the coated substrate. For example, an organic coating such as amorphous carbon can inhibit the transmission of elements including water, oxygen and carbon dioxide. Accordingly, carbon coatings have been applied to substrates, for example, polymeric films, to improve the barrier characteristics exhibited by the substrate. Another example of coatings applied to substrates to improve barrier adhesion performance includes coatings comprised of inorganic materials such as inorganic metal oxides. Ethyl vinyl alcohol and other polymer skin layers are widely used to prime or improve the wettability of film substrates for the application of a barrier layer also referred to herein as “metallization primer”. Aluminum metal, aluminum oxide, and silicon oxide are widely used for the application of barrier layer(s) directly to the substrates also referred to herein as “metallization”.
The inorganic coatings described above may be deposited onto substrates through various techniques as known in the art. Such techniques include physical vapor deposition (PVD) or chemical vapor deposition (CVD) processes. Examples of PVD include ion beam sputtering and thermal evaporation. Examples of CVD include glow discharge, combustion chemical vapor deposition (CCVD) and plasma enhanced chemical vapor deposition (PECVD) by generation of flame plasma or in strong electric fields.
The most commonly known and utilized method for depositing barrier layers on packaging substrates for metallization requires the use of a vacuum chamber to provide the vacuum environment for the deposition of inorganic atoms/ions on to the film substrate surface. This known technique, as used in the food packaging industry, consists of processing packaging substrate rolls that are from less than one to three meters wide and 500 to 150,000 meters in length running at industry speeds of 60-600 meters/min and higher in a vacuum metallization chamber. This equipment is highly specialized, requires a great deal of electrical power and requires large capital expense. Current vacuum chamber processes for metalizing films is inefficient in many respects due to the high operational costs and limited production capacity associated with the use of such equipment. Moreover, higher quality film substrates, requiring additional capital expenditure, must typically be used to achieve the desired barrier properties.
Combustion chemical vapor deposition (CCVD) and plasma enhanced chemical vapor deposition (PECVD) apparatus and methods are known in the art, as disclosed in U.S. Pat. Nos. 5,997,996 and 7,351,449, the disclosures of which are hereby incorporated by reference. Typically, a combustion flame or plasma field provides the environment required for the deposition of the desired coating (via the vapors and gases generated by the combustion or plasma) onto the substrate. The elemental precursors (e.g. organometallics) may be vaporous or dissolved in a solvent that may also act as a combustible fuel. The deposition of organic and inorganic oxides may then be carried out under standard and/or open atmospheric pressures and temperatures without the need of a vacuum chamber, furnace and/or pressure chamber.
As described above, the application of barrier to food packaging is required to protect food and food products from the effects of moisture and oxygen. It is well known in the art that metalizing a petroleum-based polyolefin such as OPP or PET reduces the moisture vapor and oxygen transmission through specialty film by approximately three orders of magnitude. Conventional technology employs an inorganic layer of metal or ceramic on a specialized polymer film. The inorganic layer may be aluminum, silicon, zinc, or other desired element in a metal or oxide form. However, the surface of the substrate onto which the barrier layer will be applied is typically primed to increase its surface energy so as to be receptive to the metal barrier to be deposited thereon and/or to “smooth” the surface to be metalized so as to reduce the surface gauge variation or surface roughness of the film to be metalized. The term “wettability” is defined herein to include surface energy, metal adhesion bond strength, and any other associated characteristic that would increase the receptiveness of the film layer surface for deposition of an inorganic ultra-thin as disclosed herein.
For example, the utilization of aluminum metal as a barrier layer on low energy plastics, such as biaxially oriented polypropylene (BOPP) film, requires a metallization primer to reduce the gauge variation of the film substrate surface and/or to improve the adhesion or bond between the metal and film substrate. Various chemical methods are employed to prime the substrate surface layer for improving the substrate surface and/or bonding of the metal barrier layer to the film substrate. With polymer film substrates, one method to prime the substrate for metallization is to co-extrude a specialized polymer as a skin layer on the substrate film. These skin layers may comprise ethyl vinyl alcohol (EVOH), polyvinyl alcohol (PVOH), and polyvinyl acetate (PVA), ethyl vinyl acetate (EVA), polyethylene terephthalate glycol (PETG), amorphous polyethylene terephthalate (aPET), among other polymers used in the industry. Unfortunately, these materials are quite expensive and add additional cost to the manufacture of metallization ready films. Plastic film cores, such as oriented polypropylene (OPP), polystyrene (PS), and polyethylene terephthalate (PET) are typically treated with corona discharge or flame treatment. However, these treatments tend to create undesired, adverse impacts on film substrate characteristics such as the formation of pin holes, chemical degradation of the surface through cross linking or intra-molecular chain scission that can adversely affect downstream metallization and heat sealing processes.
As such, there exists a need for an improved apparatus and method for depositing an ultra-thin inorganic oxide layer onto a packaging substrate to prime a substrate for metallization. Likewise, a need exists in the art for an improved apparatus and method for depositing multiple ultra-thin layers of an inorganic oxide layer on to a packaging substrate to enhance the barrier properties of a packaging substrate, which is less expensive and more energy efficient than tradition metallization while achieving and maintaining high quality barrier characteristics.